Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
The cost and conversion efficiency of LEDs are important factors in determining the rate at which this new technology will replace conventional light sources and be utilized in high power applications. Many high power applications require multiple LEDs to achieve the needed power levels, since individual LEDs are limited to a few watts. In addition, LEDs generate light in relatively narrow spectral bands. Hence, in applications requiring a light source of a particular color, the light from a number of LEDs with spectral emission in different optical bands is combined. Thus, the cost of many light sources based on LEDs is many times the cost of the individual LEDs.
The conversion efficiency of individual LEDs is an important factor in addressing the cost of high power LED light sources. The conversion efficiency of an LED is defined to be the ratio of optical power emitted by the LED to the electrical power dissipated. Electrical power that is not converted to light in the LED is converted to heat that raises the temperature of the LED. Heat dissipation places a limit on the power level at which an LED operates. In addition, the LEDs must be mounted on structures that provide heat dissipation, which, in turn, further increases the cost of the light sources. Thus, if the conversion efficiency of an LED can be increased, the maximum amount of light that can be provided by a single LED can also be increased, and hence, the number of LEDs needed for a given light source can be reduced. In addition, the cost of operation of the LED is also inversely proportional to the conversion efficiency. Hence, there has been a great deal of work directed to improving the conversion efficiency of LEDs.
For the purposes of this discussion, an LED can be viewed as having three layers, the active layer sandwiched between two other layers that form a p-i-n diode structure. These layers are typically deposited on a substrate such as sapphire. It should be noted that each of these layers typically includes a number of sub-layers.
Improvements in materials have led to improvements in the efficiency of light generated in the active layer. GaN based LEDs have shown particular promise in terms of increased light conversion efficiency with respect to the active layer. Unfortunately, these materials have a very high index of refraction, and hence, light may be trapped within the LED due to internal reflection at the air-LED boundary or other boundaries at which light traveling in a material of high index of refraction encounters a material of a lower index of refraction. A significant fraction of the trapped light is lost due to absorption in the GaN materials. In the case of conventional GaN-based LEDs on sapphire substrates, approximately 70% of the light emitted by the active layer remains trapped within the LED unless the simple planar layer LED structure is altered.
Several techniques have been described to improve light extraction from LEDs by minimizing this type of internal trapping. One class of techniques operates by randomizing the angles with which light rays that strike an internally reflecting boundary are reflected from that boundary. If an LED is constructed from parallel planar layers, a ray that strikes a first boundary at an angle greater than the critical angle at an interface between two layers of different indices of refraction will be reflected internally toward a second boundary. This ray will strike the second boundary at the same angle at which the ray struck the first boundary, and hence, be reflected back toward the first boundary at the same angle. Hence, the ray is trapped between the boundaries and will either eventually be absorbed by the materials or exit through a side surface of the LED. By roughening one of the boundaries, the correlation between the angle at which a ray strikes a boundary on a first reflection and the angle at which that ray strikes that boundary on a second reflection is significantly reduced, or eliminated. Hence, each time a ray strikes a boundary, the ray has a finite chance of escaping even if that ray was reflected internally at the previous reflection of that ray.
However, there are limits on the degree of roughening that can be achieved without introducing other problems that impact the efficiency of the LED through other mechanisms. The degree of roughening that can be obtained though shaping of the top surface of the LED is limited by the thickness of the p-layer, which is usually the layer that is exposed after the layers are fabricated. The p-layer material has a significantly higher growth temperature, and hence, the thickness of this layer must be held to a minimum during epitaxial growth to prevent degradation of the underlying active layer. However, the layer must be thick enough to allow the layer to be roughened sufficiently to scatter light that strikes the layer. The scattering structures must have dimensions that are of the order of the wavelength of the light that is being scattered to provide efficient scattering. Hence, the p-layer must have a thickness that can be roughened to provide features of the order of the wavelength of the light generated in the active layer and still remain intact. This trade-off limits the degree to which the internal reflection problem can be addressed by roughening the surface of the LED.
Another set of techniques that addresses the issue of light trapping involves providing a curved surface at the face of the LED through which light exits. If the curved surface is large compared to the LED, light leaving the LED at any angle will encounter the surface at an angle less than the critical angle and escape. This solution is typically applied at the packaging level by covering the LED with a layer of material that has a convex surface. The layer of material, however, is constructed from a material that has an index of refraction that is significantly less than that of the GaN layers from which the LED is constructed. Hence, while light that is emitted into this interface layer escapes with high efficiency, light is still trapped in the LED due to internal reflections at the boundary of the interface layer and the LED.
In addition, this solution requires a surface that is much larger than the LED, and hence, increases the size of the light source. Furthermore, the interface structure complicates the packaging of the LED in that it requires the light source manufacturer to mold the interface structure as part of the packaging operation.